When a motor operates, a part of electrical energy is converted into mechanical energy for being outputted, and a part of electrical energy is converted into heat energy to slowly spread to the air or other medium through materials of the motor structure. If the heat energy can not be transferred in time to the medium other than the motor, the temperature of the motor winding will rise, and when the temperature reaches a certain level, the motor is possible to burn out.
The common method for detecting the temperature of the motor winding is called as “over-current protection method”, in which the running current of the motor is detected and when the running current reaches a specified value a specified action is executed for protecting the motor. Since the running current of the motor is not the only factor that causes heating and temperature rising of the motor winding and the growth speed of the temperature of the winding also relates to the thermal characteristic and load of the winding, the “over-current protection method” has shortcomings.
In addition, a more appropriate method is to embed a thermosensitive element in the dangerous heat source portion inside the motor to directly detect the temperature of heat source of the motor; and when the temperature exceeds the allowable value, actions related to the thermal protection of the motor will be started. This method has the advantage of accurately detecting temperature, but it needs to add an auxiliary device which has a high cost. Furthermore, for some small-sized motors, the installation of the thermosensitive element is too complex. In addition, the actual temperature of the dangerous heat source can not be detected unless the thermosensitive element sufficiently approximates to the dangerous heat source. However, for the motor, this is difficult to achieve.